A light emitting diode (LED) is a semiconductor light-emitting device which converts electrical energy into ultraviolet light or visible light to emit it, and has advantages that it has a long life and high reliability, and when it is used as a light source, its replacement works are reduced. A LED lamp having a LED chip sealed with, for example, a transparent resin is used extensively for a backlight of a liquid crystal display which is used for a display unit of portable communication devices, PC peripheral devices, office automation equipment, home electric appliances and the like, and for signal equipment, various types of switches, vehicle-mounted lamps, and illuminating devices such as general lighting and the like.
A color tone of light emitted from the LED lamp is not limited to an emission wavelength of the LED chip, but light in a visible light region ranging from blue to red can be obtained for usage by, for example, applying phosphors to the surface of the LED chip or containing phosphors into the transparent resin for sealing the LED chip. Especially, a white light-emitting LED lamp (white LED lamp) is spreading quickly for usage as the backlight of a display unit of portable communication devices, vehicle-mounted lamps and the like, and its use is expected to be increased drastically as a replacement for the fluorescent lamp in the future.
At present, as the white LED lamps which are becoming popular or being tried and tested, there are known a LED lamp which combines a blue light-emitting LED and a yellow light-emitting phosphor (YAG or the like), and a LED lamp which combines an ultraviolet emission LED having an emission wavelength of 360 to 440 nm and a mixture of individual phosphors of blue, green and red (three-color mixed phosphor/BGR phosphor). At present, the former white LED lamp using the blue light-emitting LED is spreading more quickly because it excels in luminance characteristics to the latter.
But, the former white LED lamp has disadvantages that it looks yellowish depending on a direction, and an uneven yellow or blue color appears when projected on a white surface. Therefore, sometimes the former white LED lamp is also called pseudo-white. An average color rendering index indicating the quality of white light of the former white LED lamp is also limited to a range of 70 to 75.
Meanwhile, the white LED lamp using the latter ultraviolet emission LED is inferior in luminance to the former, but nonuniformity in light emission as well as projection light is slight, and it is expected to become mainstream of the white lamp for lighting in the future and being developed quickly. The white LED lamp using the above ultraviolet emission LED has the color rendering properties and the lamp characteristics such as luminance affected by the characteristics of the individual phosphors of blue, green and red and also the combinations of the phosphors. Therefore, the selection and combination of the individual phosphors of blue, green and red are now under consideration.
For example, Nonpatent Literature 1 describes a white LED lamp which combines an ultraviolet emission LED and a BGR phosphor. Here, a Eu-activated halophosphate phosphor or a Eu-activated aluminate phosphor is used as a blue light-emitting phosphor, a Cu and Al-activated zinc sulfide phosphor or a Eu and Mn-activated aluminate phosphor is used as a green light-emitting phosphor, and a Eu-activated yttrium oxysulfide phosphor is used as a red light-emitting phosphor.
Patent Literature 1 describes that a white LED lamp provided with an ultraviolet emission LED uses a Eu-activated halophosphate phosphor or a Eu-activated aluminate phosphor as a blue light-emitting phosphor, a Eu and Mn-activated aluminate phosphor as a green light-emitting phosphor, and a Eu-activated lanthanum oxysulfide phosphor as a red light-emitting phosphor.
A conventional white LED lamp has high color rendering properties and the uniformity of light emission which are features of the lamp using an ultraviolet emission LED, but its luminance characteristics are insufficient, and further improvement is being requested. To achieve both the high color rendering properties and high luminance by the white LED lamp using the ultraviolet emission LED, it is necessary that light at around 450 nm, around 560 nm and around 620 nm, where there are peaks of human color sensitivity in terms of the white light spectrum, are included in a good balance, and the individual phosphors of blue, green and red luminescent components are well balanced in luminous efficiency.
But, among the individual phosphors used for the conventional white LED lamp, the red light-emitting phosphor is poor in luminous efficiency with respect to the ultraviolet light or purple light at a wavelength of 380 nm or more in comparison with the other phosphor, so that it has become apparent that the luminance characteristics of the white LED lamp cannot be improved sufficiently. The properties of the blue and green light-emitting phosphors cannot be exerted enough because of the red light-emitting phosphor which is poor in luminous efficiency, resulting in deterioration of the luminance characteristics. Nonpatent Reference 1: Mitsubishi Cable Industries Review, No. 99, July 2002
Patent Reference 1: JP-A 2000-073052 (KOKAI)